1. Field of the Invention
The invention relates to an exhaust manifold and, more specifically, to an exhaust manifold that includes a double collecting pipe that guides exhaust gas from exhaust ports of a set of cylinders.
2. Description of the Related Art
Generally, an internal combustion engine (engine) has a catalyst arranged in an exhaust passage to purify exhaust gas. However, when the temperature of the catalyst is lower than its active temperature, it is difficult to ensure favorable exhaust gas purification performance. Therefore, it is necessary to promptly heat the catalyst to the active temperature, for example, when the engine is started.
Japanese Patent Application Publication No. 7-224649 (JP-A-7-224649) describes a double exhaust pipe. The double exhaust pipe includes an inner pipe and an outer pipe that are arranged via a heat insulating layer, such as an air layer. Exhaust gas flows through the inner pipe.
In the above double exhaust pipe, the outer pipe ensures structural strength, and the thickness of the inner pipe that constitutes an exhaust gas passage is reduced to make it possible to decrease the heat capacity of a portion that contacts exhaust gas. In addition, the heat insulating layer is provided between the inner pipe and the outer pipe, so it is possible to reduce radiation of heat through the outer pipe.
Thus, when the engine is started, the temperature of an inner wall of the exhaust manifold may be quickly increased. Hence, the effect of insulating heat of exhaust gas is improved to make it possible to quickly heat the catalyst to the active temperature.
In addition, Japanese Patent Application Publication No. 10-252457 (JP-A-10-252457) describes an exhaust manifold of this type. Upstream pipes are respectively connected to exhaust ports of an engine. Each of the upstream pipes has a double pipe structure formed of an inner pipe and an outer pipe. The exhaust manifold includes a double collecting pipe that collects a set of the upstream pipes, each of which is formed of the double pipe. The shapes of these upstream pipes and double collecting pipe are simplified to reduce the size of the exhaust manifold.
The double collecting pipe is formed of a common inner pipe and a common outer pipe. The common inner pipe includes a collecting pipe portion and branch pipe portions that are bifurcated from the collecting pipe portion. The common outer pipe covers an outer peripheral portion of the common inner pipe with a certain gap from the common inner pipe. Exhaust gas exhausted from the set of exhaust ports is collected at the double collecting pipe. Thus, the shapes of these upstream pipes and double collecting pipe are simplified to make it possible to reduce the size of the exhaust manifold.
In addition, the double collecting pipe is formed so that the common inner pipe is directly welded to the common outer pipe to attach the common inner pipe to the common outer pipe.
However, in the exhaust manifold described in JP-A-10-252457, the common inner pipe is welded to the common outer pipe. However, a difference in thermal expansion increases between the thin common inner pipe exposed to high-temperature exhaust gas and the thick common outer pipe exposed to outside air. Thus, the common inner pipe deforms against the common outer pipe.
That is, when the thin common inner pipe deforms at a high temperature, the thick common outer pipe does not follow the deformation of the common inner pipe. Therefore, stress at the branched portion of the common inner pipe increases, and the branch pipe portions of the common inner pipe get close to each other. This causes a deformation such that a portion between the branched portions of the branch pipe portions lifts. For this reason, the branched portion of the branch pipe portions forms cracks and is damaged, thus decreasing reliability of the exhaust manifold.